The Floors

The Structural System of the Twin Towers

This illustration from FEMA's report shows a section of the flooring system.
The main double trusses, of which two are pictured,
are perpendicular to the view plane.

The floors of the Twin Towers completed the
structural system
whose main elements were the
core structures
and the
perimeter walls.
The floor diaphragms were annular structures that spanned
the distance between the core structures and the perimeter walls,
providing large expanses of uninterrupted floor space.
The cores had their own flooring systems,
which were structurally independent of the surrounding floor diaphragms.

The 10 cm thick concrete slabs were apparently
a lightweight form of concrete typically used in high-rises.
Its density and exact composition remain unknown,
but such lightweight concrete is typically 60% as dense
as concrete used in roads and sidewalks.
The floors were the only major part of these mostly steel buildings
that contained concrete.

Some Floors Had More Than Trusses

FEMA's report
implies that the floor diaphragms were supported only by
the webbed trusses described here.
It gives no indication of other structures that may have helped
transfer the substantial lateral forces due to wind loading
between the perimeter walls and core structures,
and it provides no detail on the flooring system in
the towers' cores, which were apparently supported by heavy steel I-beams.
This idea that all the floors were undergirded only by trusses
is a prerequisite to the truss-failure theory,
which blames a chain-reaction of failures of the trusses
for the building collapses.
There is evidence, however, that certain floors had solid steel-frame
support structures rather than light open trusses,
such as the following passage from the Engineering News-Record:

On the 41st and 42nd floors, both towers will house mechanical equipment.
To accommodate the heavy loads, the floors are designed as structural
steel frame slabs. All other floors from the ninth to the top
(except for 75 and 76, which will also carry mechanical equipment)
have typical truss floor joists and steel decking.
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Truss-to-Column Connections

Most variants of the
truss-failure theory
blame the detachment of the truss ends from the perimeter walls
for the pancaking of floors, and eventual total collapse of the buildings.
It has been asserted that only one or two bolts connected the truss ends
to the perimeter walls.
However, according to both of the official government reports,
the truss ends rested on steel plates
that were both welded and bolted to the top chords of the trusses
and attached via bolted damping units to their lower bottom chords.

This illustration from FEMA's report shows details of truss-to-column
connections.